Seems as though it would make things even more secure.
A 2-of-2 multisig is more secure, but it comes with several downsides as well.
First of all, the entire security comes from the fact that both devices can't be compromised at the same time. If you store them both in your house, for example, then you have lost a significant amount of the security that a 2-of-2 multisig brings. Storing them geographically separate is more secure, but also far more inconvenient when you want to make a transaction.
Secondly, it is more expensive. Multisig transactions are larger than non-multisig transactions, and so you will pay a larger fee on every transaction you make using such a set up.
Thirdly, you need to have double the back ups, again in geographically separate locations. Do you have enough secure locations to keep all these things? If not, then again you are looking at more cost to start renting safe deposit boxes.
What's the purpose of having 2 hardware wallets yet having the same seed?
As a back up, so if one device breaks/is lost/is stolen, I don't have to restore my seed phrase on to a less secure software wallet.
I can leave one at home and carry one with me.
I can leave one at home and one at work so I can access my coins from both locations.
One for me and one for my spouse/child/business partner/etc.
There are lots of reasons you might want the same seed on two hardware wallets.